1. Field of the Invention
The present invention relates to image forming apparatuses that read an original and form an image of the original that has been read and to color shift correction methods thereof.
2. Description of the Related Art
In recent years, in order to increase the speed of image formation, the same number of developing devices and photosensitive members as the number of coloring materials have been provided in electrophotographic method color image forming apparatuses, and there has been an increasing number of color image forming apparatuses of a method (tandem method) in which images of different colors are transferred successively onto an image carrying belt and onto a recording medium.
By using this method, throughput can be greatly increased, but on the other hand color shift is produced originating in poor uniformity or accuracy of installment positioning of the lenses of a deflection scanning apparatus, or accuracy of positioning in assembling the deflection scanning apparatus itself to the image forming apparatus main unit.
That is, tilting or curvature is produced in the scanning lines and the extent of this varies for each color, thereby producing a problem of color shift due to positional shifting of each color on the transfer paper, and as a result it becomes difficult to achieve high quality color images.
On the other hand, since tilting occurs in an original itself due to the position in which the original is placed on the original platform in original reading sections of color image forming apparatuses, it is necessary to perform correction for each original at the time of reading, which necessitates a considerable processing capacity.
As a method for countering this color shift, Japanese Patent Laid-Open No. 2002-116394 for example describes a method in which an optical sensor is used at an assembly step of the deflection scanning apparatus to measure a magnitude of bending in the scanning lines, and the lens is fastened after it is mechanically rotated to adjust the bending of the scanning lines.
Furthermore, Japanese Patent Laid-Open No. 2003-241131 describes a method in which the magnitude of tilting of the scanning lines is measured using an optical sensor at a step of installing the deflection scanning apparatus to the image forming apparatus main unit, and the deflection scanning apparatus is mechanically tilted to adjust the tilt of the scanning lines when installing to the apparatus main unit.
Here, in correcting an optical path of an optical system, it is necessary to mechanically operate a correction optical system including a light source and an f-theta lens, and mirrors or the like on the optical path, then align the positions of test toner images. For this reason, the methods described in Japanese Patent Laid-Open No. 2002-116394 and Japanese Patent Laid-Open No. 2003-241131 require high-precision moving components, and this incurs much higher costs.
Further still, optical path corrections in optical systems require time until correction is completed, and therefore although it is impossible to carry out corrections frequently, the shifting in the optical path length changes by being affected by temperature increases or the like in the mechanical units. For this reason, the influence of temperature increases in the mechanical units cannot be eliminated even though corrections are carried out at a certain point in time, and therefore it is difficult to prevent color shift by correcting the optical path of the optical system.
On the other hand, Japanese Patent Laid-Open No. 2004-170755 describes a method in which an optical sensor is used to measure magnitudes of tilting and curvature in the scanning lines, then bitmap image data is corrected so as to offset these and form a corrected image thereof. This method performs corrections electrically by processing the image data, and therefore it can handle color shift at a lower cost than the methods described in Japanese Patent Laid-Open No. 2002-116394 and Japanese Patent Laid-Open No. 2003-241131 in that mechanical adjustment members and adjustment steps during assembly are not required.
Furthermore, in Japanese Patent Laid-Open No. 08-85237, image processing such as color processing and halftone processing is carried out, and raster image data is formed in a bitmap memory for each color component (C, M, Y, and K). After this, output coordinate positions of the image data for each color are automatically converted to output coordinate positions in which registration shifting has been corrected. And a configuration is disclosed in which positions of an optical beam modulated by a modifying means based on the converted image data for each color are modified by amounts smaller than the unit of the smallest dot of the color signal.
However, in electrical color shift corrections, which are one method for handling color shift, it is necessary to execute the color shift corrections after executing skew corrections at the reading section side for an original that has been placed diagonally (hereinafter referred to as “skewed original”) in the original reading section. However, in ordinary skew corrections, the skew amount varies for each original reading, which is a problem in that high speed processing capabilities are necessary for calculating the tilting.
Furthermore, by doubling the processing with skew correction processing and color shift correction processing, a possibility is sufficiently anticipated that deterioration in correction accuracy and reductions in calculation processing will occur. Further still, there is a problem in that apparatuses without a skew correction function cannot execute this.
Furthermore, techniques are conceivable of supplementing electrical color shift corrections in devices having a skew correction processing function, however these are difficult to realize since skew corrections are uniquely determined for RGB originals, while the correction amounts vary for each color of CMYK in regard to color shift corrections.